1. Field of the Invention
The present invention relates to a precision balance which includes a weighing dish supported by a support member. The support member is guided by means of guide arms in such a way that the weighing dish performs essentially only a vertical movement during weighing. The precision balance further includes an adjusting device for adjusting the vertical spacing between the ends of two guide arms. The frame of the precision balance defines a slot whose width can be adjusted by means of the adjusting device.
2. Description of the Prior Art
In high-resolution balances, the relative parallel alignment of the guide arms is of important significance. In order to avoid weighing errors when the material to be weighed is not placed in the center of the weighing dish, it is necessary to be able to exactly adjust the vertical spacing between the ends of the guide arms.
An adjusting device for the exact adjustment of the spacing between two ends of guide arms placed one above the other is disclosed in German Offenlegungsschrift No. 26 37 539. The known device includes a first adjusting device for the coarse adjustment of the spacing between the two guide arm ends, for example, during the assembly. After the balance has been assembled, a second adjusting device is used for the final fine adjustment by minimally bending the fastening surface of one of the guide arm ends. The bending moment acting on the guide arm end is small and cannot substantially impair the weighing accuracy.
Balances of modern design tend to have a low structural height. In these types of balances, the adjustment of the spacing between the ends of the guide arms is effected by means of a single adjusting device. This adjusting device includes a slot provided in the frame of the balance underneath the support and fastening points of the ends of the guide arms. As a result, elastically bending support arms are formed above the slots. Each arm and, thus, the support surface of the guide arm end can be vertically adjusted by means of a differential screw by widening or narrowing the width of the slot.
The forces originating from the differential screw are not uniformly transmitted to the arm. Due to the bending of the arm and the resulting inclination of the axis of the threaded bore, the forces act eccentrically relative to the axis of the differential screw in the bore. Consequently, the support surface is not only moved parallel to the bottom side of the slot but is also bent or tilted transversely thereof. For securing and locking the adjustment effected by means of the differential screw, frequently another screw is provided parallel to the differential screw on the arm or on the frame, wherein the end of the other screw can be brought into contact with the frame or the arm. This other screw or fastening screw causes an additional change in the position of the support surface on the arm and, thus, a change of the bending line in the arm.
This indeterminate position of the support-due to the bending during adjustment to compensate for the case when the material to be weighed is not placed in the center of the weighing dish-results in undesirable weighing errors.
It is, therefore, the object of the present invention to overcome the disadvantages of the balances described above.
In particular, it is an object of the present invention to provide an adjusting device in the precision balance in which a clearly defined bending line can be obtained in the case of elastic bending of the arm.